Problem is growth rates. Oscars grow fast, bichers do too, but only until they're about 6 inches. If you get both at three inches, the oscar's gonna grow big enough and fast enough to wonder whether the bicher is edible. If you get a bicher at 6" when the oscar is only 3", that probability diminishes, but the possibility of the bicher taking it for a deathroll increases, although as long as the bicher is well fed(and isn't an endli), that possibility decreases too. So I would probably start with larger bichers than oscars and just make sure the bichers were fat and happy.